Method for forming a metallic coating on an object



y 967 KLAUS"E)IETER ZIMMERMANN 3,318,726

METHOD FOR FORMING A METALLIC COATING ON AN OBJECT Fil6d Oct. 15, 1963 INVENTOR. K Lin/.1 zxmmexmnu BY Z United States Patent 2 Claims. 61. 117-412 Whenever metallic objects are partly immersed in a bath of liquid metal the difiiculty arises that the liquid metal not only causes a desired wetting of the surface area of the metal part but the liquid metal, due to its high surface tension, also ascends and therefore extends undesirably over contiguous surface portions of the workpiece since the undesirable wetting was hitherto unavoidable, it was necessary for the areas at which the wetting was undesirable to be cleaned by means of a troublesome after-treatment. Another possibility of avoiding the wetting at undesirable areas consists in covering these areas. However, this covering must again be removed after- Wards.

The thickness of the layer to be provided may be varied in the usual manner by varying the viscosity of the melt, for example, by adding foreign matter. However, by the conventional method of immersion, it cannot be avoided that the metallized layer on the object acquires an uneven thickness and drops are even formed in the layer.

Furthermore the oxides floating on the metal melt could not hitherto be prevented from adhering to the object surfaces to be metallized. These impurities adhering to the metallized surface may give rise to functional interference in the subsequent working process, more particularly in the metallization of semi-conductor component parts.

The disadvantages noted above are avoided in a simple manner by means of a device which is characterized by a porous netting-like or mat-like assembly. The liquid metal holds itself of its own accord due to its surface forces in the pores of the mat and again, due to surface tension, the surfaces of the objects to be metallized are coated when brought into contact with the mat. For example, a wire gauze or a perforated sheet may be used as the porous mat-like assembly. For absorbing the liquid metal, the mat-like assembly may either cover the surface of the metal bath or be pulled through a metal bath. If the mat-like assembly covers the surface of a bath of liquid metal, the surface forces cause the liquid metal to ascend into the pores of the mat and these liquid metal pores causes wetting of the workpiece surface to be metallized when the workpiece is pulled or slid over the mat-like assembly filled with liquid metal and vice-versa. The degree of wetting of the contiguous surfaces which are not to be metallized, is so slight that this undesired metallization need not be given any further consideration. If the porous mat-like assembly is drawn through a metal bath, the pores in it are likewise filled with liquid metal. In this case the mat-like assembly may be wiped or slid over a surface of an object to be metallized, during which process the liquid metal contained in the pores covers this surface. In either case the mat-like assembly serves as a kind of a wetting sponge.

A layer thickness which is even and defined may be adjusted by a suitable choice of the width and porosity of the mat. Apart from the fact that the free surface of the liquid metal when covered by a mat-like assembly according to the invention is considerably reduced and hence the formation of oxide is decreased, it has been found that in practice no oxide residues adhere to the surface to be metallized.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing, in which:

FIGURE 1 shows the wetting condition in a conventional method of metallizing metal parts:

FIGURE 2 illustrates the process of metallization by means of a device according to the invention, and

FIGURES 3 and 4 show a variant of a device according to the invention.

A receptacle 1 contains a liquid metal 3, for example tin. When a surface 5 of a workpiece 7 is brought into contact with the surface of the said tin bath, then due to its high surface tension, this bath causes not only wetting of the surface 5, but also of large portions of a contiguous surface 9 of the workpiece which is not desired to be wetted.

In FIGURE 2 the surface 11 of the bath is covered by a wire gauze 13. Pores 15 of the wire gauze 13 are filled with liquid metal which ascends into the pores 15 due to its surface tension. On placing the workpiece 7 on the wire netting 13, the liquid tin ascends, but slightly, on the sides 9 of the workpiece which are not to be metallized, since the surface of the tin bath 3 is inhibited in this case and can no longer move freely. For best results, the workpiece 7 after being placed on the wire netting 13 is pulled across it. The thickness of the metal coating depends on the pore size of the wire netting 13. As compared with the conventional method shown in FIGURE 1, no drops are formed on the covering layer in metallizing by means of the wire netting 13 according to the invention and slag parts floating at the surface of the bath 3 do not find their way into the covering layer. Such slag parts are pulled away from the surface 5 to be metallized by pulling the object over the wire netting 13. It is not necessary for the Wire netting 13 to be stretched directly over the surface of the metal bath 3 as illustrated in FIG. 2. The wire netting may alternatively be pulled through the metal bath 3 as shown in FIG. 4. After the pores 15 of the wire netting 13 are filled with liquid metal. The surface tension prevents the metal from flowing out of the pores. The wire netting 13 subsequently slides way over the surface 5 to be metallized of the workpiece 7 and, a certain proportion of the liquid metal contained in the pores 15 remains on the surface 5 to be metallized. The wire netting 13 may be formed, for example, as an endless band 17 which slides way over the workpiece 7 moved along on a conveyer belt 19. The process of metallizing surfaces of workpieces can thus be fully mechanized.

What is claimed is:

1. A method of metallizing a surface of a metal object comprising the steps of providing a receptacle containing a liquid metal bath, passing a thin metal porous mat into said liquid metal and allowing said liquid metal to be absorbed into the pores of said porous mat, sliding the obpect to be metallized relative to said porous mat and wetting the contacting surface of the object to be metallized with said liquid metal contained in the pores of References Cited by the Examiner said porous mat thereby metallizing said contacting sur- UNITED STATES PATENTS face and substantlally preventing the metalllzatlon of the surface contiguous to the contacting surface. 2109647 3/1938 North 117 112 2. A method for metallizing a metal object according 5 214701093 5/1949 Crowder et 1171 12 X to claim 1 wherein the porous member is passed across 27161076 8/1955 Fordyce at 117-112 the surface of said liquid metal bath for substantially 2,859,133 11/1958 olcott 117 112 reducing oxidation of the surface of said liquid metal h bath and thereafter sliding the porous member contain- ALFRED LEAVITT lma'y Examine ing liquid metal over a surface to be metallized. J. R. BATTEN, 111., Assistant Examiner. 

1. A METHOD OF METALLIZING A SURFACE OF A METAL OBJECT COMPRISING THE STEPS OF PROVIDING A RECEPTABLE CONTAINING A LIQUID METAL BATH, PASSING A THIN METAL POROUS MAT INTO SAID LIQUID METAL AND ALLOWING SAID LIQUID METAL TO BE ABSORBED INTO THEPORES OF SAID POROUS MAT, SLIDING THE OBJECT TO BE METALLIZED RELATIVE TO SAID POROUS MAT AND WETTING THE CONTACTING SURFACE OF THE OBJECT TO BE METALLIZED WITH SAID LIQUID METAL CONTAINED IN THE PORES OF SAID POROUS MAT THEREBY METALLIZING SAID CONTACTING SURFACE AND SUBSTANTIALLY PREVENTING THE METALLIZATION OF TEH SURFACE CONTIGUOUS TO THE CONTACTING SURFACE. 